In conventional air-conditioning apparatuses such as a multi-air-conditioning apparatus for a building, which is one of a refrigeration cycle apparatus, cooling operation or heating operation is carried out by circulating a refrigerant between an outdoor unit that is a heat source device disposed outdoors and indoor units disposed indoors. Specifically, a conditioned space is cooled with the air that has been cooled by the refrigerant removing heat from the air and is heated with the air that has been heated by the refrigerant transferring its heat. Conventionally, HFC (hydrofluorocarbon) based refrigerants have been commonly used as refrigerants for such air-conditioning apparatuses. These refrigerants have been made to work in a subcritical region that is a pressure lower than its critical pressure.
However, in recent years, ones using natural refrigerants such as carbon dioxide (CO2) have been proposed. Since carbon dioxide has a low critical temperature, the refrigeration cycle is carried out in a supercritical state in which the refrigerant pressure in a gas cooler on the high-pressure side exceeds its critical pressure. In this case, there is a possibility of the refrigerating machine oil flowing with the refrigerant not separating uniformly in the flow branching portion as it should, and in such a case, there is a possibility of the heat exchanging performance of the refrigeration cycle being impaired.
Further, in an air-conditioning apparatus represented by a chiller system, cooling or heating is carried out such that cooling energy or heating energy is generated in a heat source device disposed outdoors; a heat medium such as water or brine is heated or cooled in a heat exchanger disposed in an outdoor unit; and the heat medium is conveyed to indoor units, such as a fan coil unit, a panel heater, or the like, disposed in the conditioning space (for example, see Patent Literature 1).
Moreover, there is a heat source side heat exchanger called a heat recovery chiller that connects a heat source unit to each indoor unit with four water pipings arranged therebetween, supplies cooled and heated water or the like simultaneously, and allows the cooling and heating in the indoor units to be selected freely (for example. see Patent Literature 2).
In addition, there is an air-conditioning apparatus that disposes a heat exchanger for a primary refrigerant and a secondary refrigerant near each indoor unit in which the secondary refrigerant is conveyed to the indoor unit (see Patent Literature 3, for example).
Furthermore, there is an air-conditioning apparatus that connects an outdoor unit to each branch unit including a heat exchanger with two pipings in which a secondary refrigerant is carried to the corresponding indoor unit (see Patent Literature 4, for example).